This invention relates to measurement of physiopathalogical parameters, for example, body fluid pressures.
On physiopathological parameter for which there has been much interest in developing measurement techniques is intracranial pressure. Interest in measuring that parameter has become especially intense since the discovery by applicant Salomon Hakim in the mid 1960's of the normal pressure hydrocephalus syndrome. (Hakim, S.: "Some Observations on C.S.F. Pressure. Hydrocephalic Syndrome in Adults with Normal C.S.F. Pressure (Recognition of a New Syndrome)." Javeriana University School of Medicine, Bogota, Colombia. 1964, Thesis No. 957. Hakim, S., Adams, R. D. "The special clinical problem of symptomatic hydrocephalus with normal cerebrospinal fluid pressure: Observations on cerebrospinal fluid hydrodynamics.", J. Neurol. Sci. 1965, 2:307-327.)
A very early technique for measuring intracranial pressure, developed by James B. Ayer at the Massachusetts General Hospital, relied on insertion of a needle into the lumbar subarachnoid space and connection of the needle via a catheter to an externally located open bore manometer. Subsequent improvements reported in Guillaume, J. and Janny, P., "Manometrique Intracranienne Continue", Rev. Neurol. 84, 131-142 (1951) and Lundberg, N., "Continuous Recording and Control of Ventricular Fluid Pressure in Neurosurgical Practice", Acta Psychiat. Neuro. Scand., Suppl. 36 (1960) replaced the open bore manometer with a strain gauge instrument, also located externally of the patient and connected via a catheter. A further improvement was to implant a small strain gauge in the patient and pass wires through the scalp.
All of these techniques shared the difficulty that a physical connection was necessary through the skin, resulting in a serious risk of infection, a likelihood of discomfort for the patient, and a probable need subsequently for sugical removal of the implanted device.
to overcome those difficulties there have been efforts at developing sensor devices that can be implanted indefinitely in the body without any physical connections such as catheters or wires. Exemplary of those efforts is the work done at Johns Hopkins University in the mid to late 1970's. Described in Chubbuck U.S. Pat. No. 4,026,276 and Viernstein, L. and Gucer, G., "Clinical Use of Intracranial Pressure Monitor: Final Report" (April 1979), a technique was developed in which a passive resonant circuit having a resonant frequency influenced by the intracranial pressure is implanted in a hole cut through the skull. Pressure measurements are made by imposing an external electromagnetic field of variable frequency and observing the frequency at which the implanted resonant circuit absorbs energy.
Another technique on which substantial work has been done is the implantation of a device in which a radioactive material is forced from a lead chamber into an adjoining chamber to a degree dependent upon the intracranial pressure being measured. External measurement is made using a radiation detector.
Another known technique generally for measuring physiopathological parameters is to implant an active electronic circuit in the body, one capable of sending electromagnetic transmissions to an outside receiver, and to modulate the transmitted signal with information relating to the measurement.
A very recently-developed technique by Cosman described in Cosman, E. R.; Zervas, N. T.; Chapman, P. H.; "A Telemetric Pressure Sensor for Ventricular Shunt Systems", Surgical Neurology, 11, 287-294, 1979, implants a coil and movable magnetic core in an aperture in the skull, with the core being connected to a diaphragm whose interior surface is exposed to the patient's intracranial pressure. Pressure measurements are made by applying a balancing pressure to the outside of the diaphragm sufficient to move the magnetic core to a reference position within the implanted coil and then reading a gauge indicating the amount of external pressure required (core position is detected by application of an external electromagnetic field).